1. Field of the Invention
The present invention relates generally to systems and methods for measuring coverage area in wireless communications systems and specifically to systems and methods for coverage verification based upon cell border probability.
2. Background of the Present Invention
Verification of coverage plays an important role in the design of wireless communications systems. For example, when a subscriber using a mobile station in a wireless telecommunication system moves from one cell to another cell, coverage by the new base transceiver station must be verified prior to the asynchronous handover. Verifying the coverage area of the base transceiver stations is also essential to selecting the locations to place the base transceiver stations within the telecommunication system which results in the most efficient resource allocation. For example, placing the base transceiver stations too close together results in a smaller coverage area for the wireless communications system as a whole. On the other hand, placing the base transceiver stations too far apart results in a discontinuous coverage area. Therefore, placement of the base transceiver stations in locations which results in the maximum continuous area requires verification of coverage for the base transmitters in the wireless communications system.
Referring now to FIG. 1 of the drawings, an exemplary wireless network, such as a Global System for Mobile Communication (GSM) Public Land Mobile Network (PLMN) 10, will be described. The PMLN 10 is composed of a plurality of areas 12, each with a Mobile Switching Center (MSC) 14 and an integrated Visitor Location Register (VLR) 16 therein. The MSC/VLR areas 12, in turn, include a plurality of Location Areas (LA) 18, which are defined as that part of a given MSC/VLR area 12 in which a mobile station (MS) (terminal) 20 may move freely without having to send update location information to the MSC/VLR area 12 that controls the LA 18. Each Location Area 12 is divided into a number of cells 22. Mobile Station (MS) 20 is the physical equipment, e.g., a car phone or other portable phone, used by mobile subscribers to communicate with the cellular network 10, each other, and users outside the subscribed network, both wireline and wireless.
With further reference to FIG. 1, the PLMN Service Area or wireless network 10 includes a Home Location Register (HLR) 26, which is a database maintaining all subscriber information, e.g., user profiles, current location information, International Mobile Subscriber Identity (IMSI) numbers, and other administrative information. The HLR 26 may be co-located with a given MSC 14, integrated with the MSC 14, or alternatively can service multiple MSCs 14, the latter of which is illustrated in FIG. 1.
The VLR 16 is a database containing information about all of the Mobile Stations 20 currently located within the MSC/VLR area 12. If a MS 20 roams into a new MSC/VLR area 12, the VLR 16 connected to that MSC 14 will request data about that Mobile Station 20 from the HLR database 26 (simultaneously informing the HLR 26 about the current location of the MS 20). Accordingly, if the user of the MS 20 then wants to make a call, the local VLR 16 will have the requisite identification information without having to reinterrogate the HLR 26. In the aforedescribed manner, the VLR and HLR databases 16 and 26, respectively, contain various subscriber information associated with a given MS 20.
The MSC 14 is in communication with at least one Base Station Controller (BSC) 23, which, in turn, is in contact with at least one Base Transceiver Station (BTS) 24. It should be understood that the BSC 23 may be connected to several base transceiver stations 24, and may be implemented as a stand-alone node or integrated with the MSC 14. In either event, the BSC 23 and BTS 24 components, as a whole, are generally referred to as a Base Station System (BSS) 25. The BTS 24 is the physical equipment, illustrated for simplicity as a radio tower, that provides radio coverage to the geographical part of the cell 22 for which it is responsible. In ascertaining the geographical area of the cell 22, the cell 22 is assumed to be a circular area with a radius R and with the BTS 24 at the center.
Coverage is defined in terms of the cell border probability (CBP) which is the probability that the strength of a randomly selected received signal along the perimeter of cell 22 exceeds a certain threshold and the cell area probability (CAP), which is the probability that the strength of a randomly selected received signal in the cell 22 exceeds a certain threshold. Coverage is verified when the CAP exceeds a desired level, e.g., 90%. One method of estimating the CAP is to take a sample of received signal strength measurements throughout the cell 22 and computing the fraction of measurements satisfying the threshold. However, in order to acquire a statistically meaningful sample, not only must a large number of measurements be taken, but the measurements must be taken at locations spread out over the area of the cell 22. Another approach is to take samples from a single area and utilize Reudink's equation to model the entire cell 22. However, use of Reudink's equation to model the cell 22 requires the estimation of certain parameters such as the propagation constant and the intercept. Consequently, the possibility of errors in the estimation of the parameters increases the likelihood of erroneous results.
It is therefore an object of the invention to provide a system and method for verifying coverage of a cell in a manner that reduces the number of measurements that must be taken.
It is also an object of the invention that the system and method verify coverage of the cell in a manner that reduces the areas in which measurements must be taken.
It is also an object of the invention to increase the probability of an accurate resultant determination.